1. Field of the Invention
The present invention relates generally to plating baths and methods for monitoring the major constituents contained therein. More particularly, the method of the present invention relates to a voltammetric analysis technique that accurately indicates concentrations of metal ions comprising a major constituent within a plating bath containing additional codepositing constituents which would otherwise interfere with voltammetric measurement accuracy. The method can be used to maintain desired major constituent concentrations in order to ensure optimal plating bath performance.
2. Description of Related Art
A typical plating bath solution is comprised of a combination of several different electrochemical constituents. The specific constituents vary depending upon the type of plating bath, but in general can be broadly divided into what are commonly known as major constituents and trace, or minor, constituents. The major constituents are those electrochemical constituents which make up about 2 to 50 percent of the total bath weight or volume. Trace constituents, on the other hand, are present in smaller quantities, usually less than 1 percent of the total weight or volume. For example, in an acid cadmium plating bath, cadmium ions are a major constituent, and typically represent about 3 to 5 percent of the total bath weight. The acid cadmium plating bath might also contain trace constituents such as organic addition agents, degradation products and chemical contaminants, present in much smaller concentrations.
The concentration levels of both major and trace constituents are important determinants of the quality of the resultant plating deposit. Trace constituent concentrations influence certain characteristics of the plating deposit, including tensile strength, ductility, solderability, uniformity, brightness and resistance to thermal shock. Monitoring and optimization of trace constituents assumes that the major constituent concentrations within the bath are already properly set and maintained. Should the major constituents fall outside of required concentration ranges, however, the bath may fail to satisfactorily perform its plating function. It is therefore important that major constituent concentrations be regularly monitored.
Current techniques for monitoring the major constituents of plating baths typically involve removing a sample of the electrochemical solution from the plating tank for subsequent wet chemical analysis. Methods of measuring major constituent content in various types of plating baths are disclosed in K. E. Langford and J. E. Parker, "Analysis of Electroplating and Related Solutions," pages 83-100, 65-68 and 174-180. Wet chemical analysis methods such as these usually must be performed by highly skilled personnel. Specialized and costly chemical analysis equipment and supplies are required. Furthermore, the delay between drawing samples and receiving measurement results can be anywhere from several hours to several days. It is thus very tedious and expensive to monitor major constituent concentrations using currently available techniques. Moreover, the slow response time of wet chemical analysis limits the extent to which a high quality plating bath can be continuously maintained.
The current major constituent monitoring techniques are quite different from real time trace constituent monitoring techniques such as those in U.S. Pat. No. 4,631,116, assigned to the present assignee. The method disclosed therein uses voltammetric techniques to produce ac current spectra which vary as a result of changes in the concentration of various trace constituents. Voltammetric methods have been found to produce accurate results in real time for trace constituent analysis.
An important problem which may enter when voltammetric techniques are applied to major constituent analysis is that in many plating baths other trace or major constituents will codeposit on a sensing electrode with the major constituent being analyzed. For example, hydrogen ions coreduce and codeposit with metal ions to varying degrees in many types of acidic plating baths. Some voltammetric techniques rely upon a measurement of the total charge plated and then stripped from an electrode, usually in the form of a response current signal, to indicate the concentration of a desired constituent. When other constituents codeposit or are evolved with the desired constituent,, the measured charge is influenced by these codepositing constituents. Coreduction and codeposition of other constituents such as hydrogen ions on a sensing electrode surface therefore reduces the accuracy of monitoring techniques such as dc voltammetric and anodic stripping, as well as ac techniques such as those disclosed in U.S. Pat. No. 4,631,116. Until this problem is resolved, voltammetric monitoring system accuracy will be insufficient for major constituent analysis of certain plating baths containing codepositing constituents.
In the case of the acid cadmium plating bath discussed above, hydrogen ions within the bath will codeposit with cadmium ions when a voltammetric signal is applied to a sensing electrode in contact with the bath. The charge due to the codepositing hydrogen will alter the resultant response current signal and prevent accurate measurement of cadmium ion concentration.
As a result of this problem, it is presently necessary to use voltammetric trace constituent measurement techniques in conjunction with other major constituent chemical analyses in order to accurately monitor the overall quality of a plating bath containing codepositing hydrogen. The wet chemical analysis cannot be performed with the in-tank electrochemical sensors and other equipment typically used in trace constituent analysis. Two different sets of equipment must therefore be maintained in order to perform major and trace constituent analysis. No integrated measurement system is available which is capable of measuring both major and trace constituents in plating baths containing codepositing constituents.
As is apparent from the above, there presently is a need for an accurate and inexpensive real time method of monitoring the concentration of major constituents within a plating bath containing other codepositing constituents such as hydrogen ions. Furthermore, the method should complement and be easily integrated with known techniques and equipment suitable for measuring trace constituents, resulting in an efficient overall plating bath analysis system.